The art of diagnostic imaging exploits contrasting agents that in binding or localizing a site selectively within the body, help to resolve the image of diagnostic interest. 67Gallium salts, for example, have an affinity for tumours and infected tissue. With the aid of scanning tomography, 87Gallium salts can reveal afflicted body regions to the physician. Other contrasting agents include metal radionuclides such as 99technetium and 186/188rhenium. These have been used to label targeting molecules such as proteins, peptides and antibodies that localize at desired regions of the human body.
It is critical for the management of patient care to have the ability to quickly and accurately identify sites of infection and resultant inflammation. Current radionuclide labeled targeting molecules for targeting sites of infection and resultant inflammation bind to leukocytes (white blood cells). These targeting molecules do not provide prompt diagnosis due to delays of up to 12-24 hours following injection. These delays occur because the targeting molecules require the separation of leukocytes from the patents whole blood before radiolabeling. Further delays result because after re-injection the leukocytes take several hours before they can re-localize to the sites of inflammation. Other targeting molecules have high molecular weight which results in slow delivery of the radiopharmaceutical to the sites of inflammation.
The problem with time delays can be overcome by using radiopharmaceuticals that bind well to N-formyl-methionylleucyl-phenylalanine (fMLF) receptor on leukocytes. This receptor is also referred to as formyl peptide receptor (FPR). The accumulation of leukocytes at sites of inflammation is the primary mechanism by which the immune system localizes and destroys microbial and other toxic agents. Radiopharmaceuticals that bind to fMLF receptor can label leukocytes that are present in circulation as well as those at sites of inflammation. This allows for prompt diagnosis.
fMLF is a bacterial product that initiates leukocyte chemotaxis by binding to high affinity fMLF receptors present on polymorphonuclear leukocytes (PMNs) and mononuclear phagocytes. Chemotaxis refers to the migration of leukocytes to sites of inflammation and infection. Current targeting molecules that that bind well to fMLF receptor are agonists for this receptor. These targeting molecules therefore elicit an accumulation of leukocytes in healthy tissue that causes tissue damage. This effect is called neutropenia. This is a significant problem.
Most known chemotactic antagonists however exhibit low binding affinity to the fMLF receptor. None have a group that is suitable for transporting a radionuclide that is essential for radioimaging applications.
There is therefore a need for radiopharmaceuticals that have a high affinity for the fMLF receptor so that they can target leukocytes that accumulate at sites of inflammation. There is a further need for such radiopharmaceuticals that are antagonists or very weak agonists of chemotaxis in order to eliminate problems such as neutropenia that are associated with agonists.
Recent studies have indicated that the binding affinity of targeting peptides and the chemotactic functionality of those targeting peptides depend on two different factors. The three amino acid residues at the N-terminus of the peptides (methionine, leucine and phenylalanine) mainly govern the degree of binding of the targeting peptide to the receptor. There is also a contribution from amino acid residues that are more removed from this site. The functional group attached to the N-terminus of the peptides has a large affect on the chemotactic functionality of the peptide when bound to the receptor. For example the presence of a formyl group or an acetyl group has been shown to provide agonist or weak agonist activity whereas a tertbutoxycarbonyl group causes the peptide to exhibit antagonist activity but with low affinity binding.
There is therefore a need for a radiopharmaceutical that is a chemotactic antagonist with an N-terminus capping group and a structure that confers a strong binding affinity for the fMLF receptor to the radio pharmaceutical.